Depending on the model year, the Prius has 2 drain plugs. Also, the original US Prius takes 2.9 quarts, not 3. Overfilling that model is easy AND bad.
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longneckMay 10 '13 at 18:00

1

You should use the marks on the dipstick, not the amount drained, to measure how much oil you're burning/losing. When you drain the oil, some remains in the engine, and some is in the oil filter.
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R..May 10 '13 at 18:34

Try the owner's manual. If that doesn't say, contact your Toyota service department and have them check the maintenance manuals. Different cars have different acceptable burn off/losses. For example, my Toyota MR2 is within specification as long as it doesn't burn more than 1 quart every 1000 miles.